The object of the invention is a method for enriching xcex2-glucan in products obtained from dehulled or naked oats. More specifically, the object of the invention is the method for preparing an oat product enriched in the content of xcex2-glucan, in which method dehulled or naked oats are subjected to dry milling and dry fractionation at multiple stages. A further object of the invention is the use of such product as a component in foodstuffs such as cereal, meat, candy, beverage, or prepared meal products.
As it has been shown in numerous animal and clinical studies, ingesting soluble dietary fibre such as oat xcex2-glucan causes a reduction in blood total and low-density lipoprotein cholesterol content, which in turn reduces the risk of coronary heart disease. The principal mechanism of this action is according to the present view a reduction of the back absorption of bile acids in the small intestine, which lead to their increased excretion in the faeces, and correspondingly to their increased synthesis from cholesterol. Simultaneously, soluble fibre retards and weakens also the absorption of glucose in the small intestine, which leads to a diminished secretion of insulin. Consequently, synthesis of cholesterol, which is promoted by insulin, is weakened. The said retarding of the absorption of glucose is of advantage for diabetic patients or for persons having a weakened glucose tolerance. This offers them a possibility to reduce the post-meal elevation of glucose and a later hypoglycemic condition. Correspondingly, controlling fluctuations in blood glucose level is also of advantage for improving the prestation of sportsmen and in long-duration exercise. Weakening of the glucose absorption has been shown to be effected by the increased viscosity in the small intestine. There are also indirect but no direct proofs of the dependence of the reduction of cholesterol on the viscosity.
In addition to these effects, soluble dietary fibre has been shown to have several other health-promoting effects, such as alleviating several disorders of the intestine, diminishing risks of hormone mediated cancers, and in composing weight reduction diets especially in improving the control of appetite.
The use of cereal soluble fibre for functional foods has so far been limited mainly to breakfast cereals and to certain bakery products. For achieving a significant reduction of cholesterol, the daily intake of xcex2-glucan has to be at least 3 g (Department of Health and Human Services, USA, Federal Register 62, 3584-3601, 1997). For controlling fluctuations of blood glucose and insulin, the amount of xcex2-glucan needed is 5 to 6 g per meal (Tappy et al., Diabetes Care 19, 831-834, 1996). In addition to the amount, it is important that xcex2-glucan is sufficiently rapidly soluble and elevates the viscosity efficiently. Obtaining the minimal daily amount of xcex2-glucan needed for reduction of cholesterol has been possible using commercial ingredients available so far, but the amounts of oat products to be ingested daily have been so high, that only few persons can be persuaded to follow such diets regularly or for long periods. Balancing fluctuations of blood glucose and insulin by using xcex2-glucan is not possible without a remarkable concentration of xcex2-glucan from the level present in native oat grains and traditional oat products.
The content of xcex2-glucan in commercially cultivated oats is usually within the limits 2.5 to 4.5%, but can in exceptional lots be up to 5.5% of the dry weight. Oat bran produced using traditional milling and sieving techniques contains xcex2-glucan usually 5.5 to 7%, but can exceptionally contain up to 10% from the dry weight. Achieving higher contents by using dry milling methods is said to be limited by the soft structure and the fat content of oat grains. In scientific research papers there are reports of samples prepared using dry fractionation methods and having xcex2-glucan contents from 10.3 to 12.8% (Shinnick et al, Journal of Nutrition 118, 144-151, 1988, Shinnick et al., Journal of Nutrition 120, 561-588, 1990, Wood et al, Cereal Chemistry 66, 97-103, 1989, Doehlert and Moore, Cereal Chemistry 74, 403-406, 1997), but commercial production of such products has not succeeded.
In most of the research papers on dry milling concentration, the milling has been performed in one stage, after which the milled product has been fractionated by sievings and/or air classifications. Thus in the said publication of Wood et al., one stage pin milling has been used, followed by air classification with a capacity of 105 kg/h. They obtained a concentrate containing 12.8% xcex2-glucan with a yield of 34%, but reported a partial blocking of the equipment after handling a lot of 468 kg. As compared to the starting material, a 2.29-fold concentration of xcex2-glucan was achieved. Doehlert and Moore used a laboratory scale roller mill and obtained after two sieving stages a concentrate containing 11% xcex2-glucan, with a yield of 22.3%. Using impact type milling they obtained with a yield of 27.4% a concentrate having a xcex2-glucan content of 8.85%.
Dry milling in one to two stages has been used by Myllymxc3xa4ki et al. (U.S. Pat. No. 5,312,636), as the first step in their method for solvent wet milling fractionation. The content of xcex2-glucan obtained after the dry fractionation stage was 11-12%. Vorwerk (Getreide, Mehl und Brot 1990, 265-267) reported of concentration using three subsequent roller milling and sieving operations. The study has evidently been made in pilot scale, and no details have been disclosed. A concentrate containing 23% of total dietary fibre, corresponding to 11.5% xcex2-glucan, was obtained with a yield of 15%.
A more advanced concentration of xcex2-glucan is possible after removing fat with an organic solvent (Wood et al, Cereal Chemistry 66, 97-103, 1989, Knuckles et al., Cereal Chemistry 69, 198-202, 1992, and Wu and Stringfellow, Cereal Chemistry 72, 132-134, 1995), followed by dry milling and fractionation operations, or by wet milling in an organic solvent (Myllymxc3xa4ki et al., U.S. Pat. No. 5,312,636, Wood et al., Cereal Chemistry 66, 97-103, 1989, Collins et al., U.S. Pat. No. 5,169,660, Mxc3xa4lkki and Myllymxc3xa4ki, U.S. Pat. No. 5,846,590), or by wet milling in cold water (Lehtomxc3xa4ki et al., U.S. Pat. No. 5,106,640). U.S. Pat. No. 5,183,667 has been granted for an oat fibre concentrate containing 15 to 40% of xcex2-glucan. For the method of production, wet milling in cold water which may contain ethanol has been given.
For isolation of purified xcex2-glucan, alkaline extraction followed by various purification stages and final precipitation of xcex2-glucan by ethanol or ammonium sulfate have been used (Hohner and Hyldon, U.S. Pat. No. 4,028,468, Wood et al., Cereal Chemistry 55, 1038-1049, 1978, Myllymxc3xa4ki et al, U.S. Pat. No. 5,312,636, Collins et al., U.S. Pat. No. 5,169,660, Bhatty, Journal of Cereal Science 22, 165-170, 1995). The purity achieved in technical or pilot scale has been 60-80%.
As a drawback for the said methods using organic solvents is the elevation of processing costs, which limit the economical use of these fibre concentrates in food products. Also using the cold water wet milling involves considerable costs of drying the products, and in addition, in animal and clinical studies performed so far the cholesterol reducing effect of the product obtained has been weaker than with products prepared by dry milling methods (Mxc3xa4lkki et al., Cereal Chemistry 69, 647-653, 1992, Uusitupa et al, Journal of the Americal College of Nutrition 11, 651-659, 1992, Txc3x6rrxc3x6nen et al., European Journal of Clinical Nutrition 46, 621-627, 1992). In several studies it has been found, that in the isolation of xcex2-glucan its molecular weight and viscosity are reduced (Wood et al., Cereal Chemistry 66, 97-103,1989), which can lead to a complete loss of the cholesterol reducing effect (Beer et al, European Journal of Clinical Nutrition 49, 517-522, 1995). Health claims connected with xcex2-glucan are in the United States allowed so far only regarding reduction of cholesterol and reducing the risk of coronary heart disease, and only when using wholemeal oat products or oat bran, which have been produced using dry milling and separation techniques.
According to this invention it has now been surprisingly observed, that oat xcex2-glucan can be concentrated using dry milling and separation methods also in industrial scale to higher concentrations than the said 10% achieved in the industrial scale using traditional methods. The essential characteristic of the invention based on said observations are presented in the claims attached.
The method according to this invention involves a selective milling, which is performed in two main stages. In the first main stage, the milling conditions are selected with the purpose to preserve as far as possible the integrity of the outer layers of the dehulled grains, whereby the endosperm which has a high content of starch is separated as a fine powder. The coarse fraction obtained from this main stage is a preconcentrate, which is treated further in the second main stage by using more effective impact, shear or roller operations. This enables to separate the main part of the starch of the subaleurone layer from the cell wall constituents. Separation of the coarse and fine fractions can be effected either by sievings including air flushed sieves, by air classifying, or these operations after each other. Critical stages of the process are connected partly to milling, partly to separation stages.